1. Field of the Invention
The present invention relates to an inductor driving circuit for driving an inductor.
2. Description of Related Art
Generally, a solenoid having a simple structure and operable at a high speed has been used for a relay and an electromagnetic contactor. Particularly, a DC solenoid is often used from a viewpoint of easiness to handle. Here, attention should be paid on a surge generated when a power supply is turned off. When the power supplied to the solenoid is turned off, a counter electromotive voltage is generated in the solenoid, which causes generation of a surge. There is a danger that a surge may destroy a semiconductor switch or other components for controlling the power supply to the solenoid. Various measures have been proposed against such the surge, as described in Japanese Patent Application Publications (JP-A-Heisei 9-199324, related art 1; JP-P2001-132866A, related art 2; and JP-P2002-15916A, related art 3).
FIG. 1 shows an example of a driving circuit for driving a DC solenoid. A DC power supply DCPS is connected to a solenoid 100 via a switching element SW. When the switching element SW is turned on (i.e. power supply is turned on), a DC driving voltage is applied to the solenoid 100 and a DC current starts to flow. When the switching element SW it turned off. (i.e. power supply is turned off), application of the DC driving voltage stops. In the example of FIG. 1, a current circulating diode 110 is arranged in parallel to the solenoid 100. Here, the current circulating diode 110 has a cathode connected to a positive terminal of the power supply and an anode connected to a negative terminal thereof. Therefore, no current flows through the current circulating diode 110 when the power supply is turned on. However, when the power supply is turned off, a counter electromotive voltage is generated in the solenoid 100. At this time, a loop is formed by the solenoid 100 and the current circulating diode 110 and a circulation current flows as shown by an arrow in FIG. 1. Therefore, effects of a surge to the DC power supply DCPS and the switching element SW or other components are effectively reduced.
Here, energy of the circulation current generated after turning off the power supply is consumed as joule heat in all inductor (or coil) which drives the solenoid 100. Therefore, attenuation time before achieving sufficient attenuation of the circulation current is relatively long. In this case; a time from timing When power supplied to the solenoid 100 is turned off to timing when a physical contact connected to the solenoid 100 is turned off is elongated. That is, a delay in a mechanical operation to turn off the power supply is enlarged. It is not preferable from a viewpoint of operating a machine at high speed.
FIGS. 2 and 3 show other examples of the driving circuit. In the example of FIG. 2, a capacitor 121 and an attenuation resistor 122 are connected in series between the positive terminal and the negative terminal. In the example of FIG. 3, a varistor 130 is connected between the positive terminal and the negative terminal. In the examples of FIGS. 2 and 3, a relatively high voltage is generated in turning off the power supply and attenuation energy which depends of a product of the high voltage and a current is made larger. That is, a time to attenuate an inductor current after turning off the power supply is shortened. Meanwhile, it is concerned that an excessive voltage or other factors are caused to the DC power supply DCPS and the switching element SW by the high voltage.